The invention relates to a method for testing electrical loads and, more particularly, to a method for testing electrical loads in a vehicle electrical system, in which the loads, which are fed by a vehicle battery, are selectively switched on and off. The load-specific current consumption in the switched-on state is determined and assessed in each case. Such a method is particularly suitable for computer-aided functional testing of the vehicle electrical system and components.
German patent document DE-B 21 42 860 discloses a method of the above-mentioned type which uses a battery adapter having integrated therein a measuring resistor and, connected thereto, an A/D converter which is connected as a voltmeter. Provided with appropriately matched connections, the battery adapter is clamped to an exposed battery terminal, on the one hand, and, on the other hand, to the battery cable previously removed from the battery terminal. The battery adapter is connected to diagnostic computer via an input/output unit. During the functional testing, the service engineer charged with testing the vehicle activates the electrical loads in the vehicle in accordance with test specifications which are prescribed to him by the diagnostic computer via a screen. The current consumption is simultaneously measured by means of the battery adapter and compared with load-specific desired values in the diagnostic computer. A fault message is issued in the event of a significant deviation.
A similar method is described in German patent document DE 39 23 545 A1. This document provides for the temporary insertion of a precision measuring resistor (shunt) into the main current path between the battery of the vehicle and the loads, for example in the place of a previously removed main fuse. The voltage drop across the resistor is evaluated for the purpose of measuring the current. The loads are activated by a diagnostic computer which issues corresponding commands to a vehicle-internal diagnostic line via a diagnostic socket outlet.
The previously described methods require a measuring resistor on which the measurement voltage is tapped. This results in additional costs irrespective of whether this measuring resistor is installed permanently or just temporarily for measurement purposes. The installation and removal are time-consuming, particularly in the case of modern vehicle types whose fuse box and, to an even greater extent, battery are often accessible only with great difficulty. Furthermore, the reconnection of the battery cable to the battery terminal or the reinsertion of the fuse often hides additional fault sources.
A further method of the above-mentioned type which, however, circumvents the installation and removal of a measuring resistor is disclosed in German patent document DE 41 30 978 A1. In this document, the fuses connected upstream of the loads are used as measuring resistors whose voltage drops are picked-off by a test plug engaging on the fuses. The voltage drops are evaluated by an external test instrument which beforehand carries out a conventional resistance measurement at the respective fuse for the purpose of normalizing the current values. This method may be deemed to suffer from the disadvantage that reliable handling of the test plug depends on the easy accessibility of the fuse box.
Furthermore, German patent document DE 43 38 462 A1 discloses a method of the above-mentioned type in which, in order to determine the current consumption of a selectively switched-on load to be tested, use is made of the voltage drop across the load which is caused by an impressed constant current. This method suffers from the disadvantage, inter alia, that a separate constant-current source is required in addition to the battery.
There is therefore needed a method which can be used to determine the current consumption of selectively switched-on loads in the vehicle in a simple and reliable manner.
These needs are met by a method for testing electrical loads in a vehicle electrical system, in which the loads, which are fed by a vehicle battery, are selectively switched on and off. The load-specific current consumption in the switched-on state is determined and assessed in each case. In order to determine the current consumption, the battery voltage is measured, immediately before and shortly after the switching-off of the previously switched-on load and the load-specific current consumption is determined from the voltage difference.
The invention exploits the fact that every load switching instance in the vehicle electrical system produces a slight sudden voltage change at the battery. Since the method according to the invention evaluates the battery voltage or vehicle electrical system voltage directly in order to determine the current consumption, with the internal resistance of the battery serving indirectly as the measuring resistance, a separate measuring resistor provided for measurement purposes is not required. For this reason, measurement lines for picking-off a voltage drop are also eliminated. The battery or vehicle electrical system voltage can be tapped remotely from the battery, for example at a diagnostic socket outlet, and is usually monitored in any case. The adaptation outlay is therefore low and independent of the accessibility of the battery. A further advantage is that the signal voltages of the sudden changes in the battery or vehicle electrical system voltage are significantly larger than the voltage drops across a measuring resistor which are evaluated in the known methods, for which reason the method according to the present invention is more sensitive and less susceptible to interference. The method according to the invention is cost-effective and can be employed during the production of the vehicle, during after-sales service or in a self-diagnostic system integrated in the vehicle (on-board diagnosis).
In an equivalent circuit diagram, the battery essentially comprises an electrochemical voltage source, a large parallel discharge resistance and a small series internal resistance, which is designated below as battery resistance or internal resistance. When current flows, the battery terminal voltage is therefore reduced by the voltage drop across the battery resistance in comparison with the voltage of the electrochemical voltage source.
In the event of a quantitive evaluation of the battery voltage for the purpose of determining the current, it should be taken into account, however, that when current is drawn for a relatively long time, the voltage of the electrochemical voltage source also decreases non-linearly as a function of the residual capacity, the temperature and the discharge current. When the load is removed, the voltage gradually rises again due to chemical processes. This slow response of the electrochemical voltage source overlies the influence of the battery resistance and is difficult to detect.
However, if the immediate response of the battery to rapid current changes considered is then it is found that only the internal resistance of the battery is effective during this brief time. On account of the high capacity and the slowly proceeding chemical processes, the voltage of the electrochemical voltage source can be regarded as being constant at that moment.
The invention therefore provides for the evaluation only of rapid load current changes in the vehicle electrical system, since these changes bring about a proportional sudden voltage change which is essentially determined solely by the battery resistance. The current consumption of a load can be acquired in a reproducible manner, in particular, by measuring the voltage difference at the changeover instant, that is to say at the instant when the load is switched on or off.
Furthermore, the method according to the present invention advantageously provides for the evaluation only of the sudden voltage change when the load is switched off. An evaluation of the sudden voltage change when the load is switched on would be inaccurate, since some loads exhibit a pronounced transient response. For example, vehicle lamps require a high switching-on current when they are switched on, on account of the initially cold incandescent filament of the lamp. The voltage difference measured during switching on would then be greatly dependent on the time interval between the two measurements. A reproducible measurement of the battery voltage is therefore not produced until after the transient processes have died away. This also applies to evaluation of the sudden voltage change when the load is switched off. The switched-on time prior to switching off must therefore be selected to be long enough for the transient processes which are typical of some loads to be able to take place.
In an advantageous development, the measured, load-specific voltage differences are normalized by means of a reference value, which is formed by a voltage difference emerging when the battery is loaded on a trial basis with a known load. The reference value can be determined anew at any time as required, with the result that the influence, which varies slowly with time, of battery capacity, charge state and aging of the battery on the internal resistance of the battery is eliminated from the evaluation of the load-specific voltage differences.